This invention relates generally to surface acoustic wave (SAW) devices and, more particularly, to SAW devices used as radio-frequency (rf) channelizers or spectrum analyzers. A basic form of these devices was disclosed in two prior applications assigned to the same assignee as the present invention, and cross-referenced as the first two listed applications.
Although the cross-referenced applications are not necessarily prior art with respect to the present invention, it is important to understand the principles and limitations of the devices they disclose. SAW devices employ substrates of a piezoelectric material, across which elastic surface waves are propagated between sets of electro-acoustic transducers disposed on the substrate surface. The surface waves, called Rayleigh waves, have an amplitude of displacement that is largest right at the substrate surface. In a piezoelectric material, deformations induced by the waves induce local electric fields, which are propagated with the acoustic waves and extend into space above the surface of the material. These electric fields will interact with electrodes disposed on the surface of the material, to serve as electrical input and output transducers for the surface acoustic wave device.
Although most SAW devices are "in-line"devices employing a single propagation direction, SAW technology can also be applied to diffraction-effect devices, such as spectrum analyzers. SAW spectrum analyzers are disclosed in the the first two cross-referenced applications. Related principles are discussed in a paper by P. Hartemann et al., "Wave-front Synthesis and Reconstruction Using Acoustic Surface Waves," 1977 Ultrasonics Symposium Proceedings, IEEE Cat. #77CH1264-1SU, pp. 840-42. The paper also discloses the use of a tapped delay line for purposes of experimental analysis of spectrum analyzer operation.
SAW spectrum analyzers operate on a principle closely analogous to that of an optical diffraction grating, in which a beam of white light is dispersed into a continuum of separate monochromatic sub-beams. Basically, the channelizer device includes a curved array of input transducers located on a piezoelectric substrate. The transducers are effectively point sources of acoustic radiation, and can be considered to radiate circular wavefronts if the anisotropic nature of most SAW substrate materials is neglected. Wavefronts from all of the input transducer elements reach a zero-order focal point at the same time and reinforce each other. The zero-order focal point is, therefore, at the center of curvature of the input transducer array. At a first-order focal point, displaced from the zero-order focal point, wavefronts from two adjacent transducers still arrive in phase with each other, but their path lengths differ by one complete wavelength. For higher orders of diffraction, the path lengths differ by integral multiples of a wavelength. If the frequency of the signal applied to the input array is changed, the first- order focal point is shifted laterally with respect to the zero-order focal point. If a wideband input signal is applied to the input array, the focal point becomes a focal arc, and each point on the arc represents a different input frequency. Output transducers are arrayed along the focal arc, and each is responsive to a narrow band of frequencies.
If the distances between adjacent input transducers and the focus are offset by a one wavelength spacing, the device is said to operate in the first diffraction order. Operation in the second diffraction order is obtained by offsetting adjacent input transducers by two wavelengths, and so forth. Higher diffraction orders result in a narrower spread of frequency over the output transducers. Stated another way, higher diffraction orders yield a desirably higher frequency sensitivity, or smaller fractional frequency spread at the output transducers. Unfortunately, however, much higher diffraction orders require that the input transducer array be spread over a greater area of the substrate, and devices operating at high diffraction orders are impractical for this reason. Prior to this invention, channelizers or spectrum analyzers have not been able to achieve extremely narrow frequency channels, less than one per cent, with good sidelobe suppression between channels, preferably greater than 50 dB (decibels).
Accordingly, there is still a need for improvement in acoustic channelizer devices, and the present invention satisfies this need.